Woon Suk Hwang

Characteristics of Ni-P alloy electrodeposited from a sulfamate bath

Abstract The effect of H 3 PO 3 concentration on Ni–P electrodeposition from sulfamate bath and the material properties of the deposit with heat treatment were investigated. With increasing H 3 PO 3 concentration in the bath, the phosphorus content in the deposit increased, while the current efficiency slowly decreased and stress in the deposit increased. This result seems to be related to the increase of hydrogen evolution and absorption reactions accompanied by the increase of phosphorus content in the deposit. The size of the grains and the precipitates in Ni–1.52 wt.% P deposit rapidly increased during heat treatment at temperatures equal to or higher than approximately 490 °C, fairly consistent with the results of Vickers hardness tests. The microstructural and mechanical property variation of the deposits with heat treatment were explained by precipitation of Ni 3 P at 310 °C and recrystallization at 575 °C with the supplemental help of DSC (differential scanning calorimetry) results.

Explosion synthesis of Ti5Si3-Cu intermetallic compound

Abstract Synthesis of an intermetallic compound based on Ti 5 Si 3 by an explosive compaction of elemental powders was studied with the detonation velocity as the main experimental variable. Prior to the explosion experiment, a computer simulation of the compaction process was conducted by using the DYNA program, and the result was utilized in designing the experiment. The relative density of the compacted compound increased with the detonation velocity. To enhance the density of the compound further, however, it was necessary to adjust other variables associated with the can, powder, and the backup tube. From an X-ray diffraction analysis of the explosion-compacted compound, it was confirmed that formation of the Ti 5 Si 3 phase was complete. Although there is a room for further improvement of the density and the crack resistance of the compacted alloy, the present work verified that explosion synthesis is a potentially viable method to consolidate intermetallic compounds.

Effects of Heat Treatment on Hardness of Nanocrystalline Ni-W Electrodeposits

The effects of W content and heat treatment on the hardness of Ni-W alloy deposits were investigated by XRD and Vickers microhardness test. The surface characteristics were also evaluated with respect to the surface roughness and glossiness. With increasing W content, the surface glossiness increased, but the surface roughness decreased. And the amorphous Ni-49wt.% W and 58wt.%W deposits had higher glossiness and lower surface roughness than crystalline Ni-W deposits. The hardness of Ni-W alloy deposits increased with increasing W content because of the solid solution hardening and grain size refinement. As the heat treatment temperature was increased, the hardness increased to reach the maximum of 1300VHN at 600°C for Ni-58wt.%W deposit. The precipitation of intermetallic compounds such as Ni 4 W and NiW seemed to contribute to the increase of hardness. However, the hardness decreased at higher temperature than 600°C owing to the precipitation of pure W, which was supersaturated in Ni matrix.

Galvanic coupling effect on corrosion behavior of Al alloy-matrix composites

Galvanic coupling effect on the corrosion of SiC-reinforced aluminum alloy-matrix composites was investigated in a sodium chloride solution. The potentiodynamic polarization measurement indicated that pitting potentials of metal matrix composites (MMCs) and AA2124 matrix alloy were similar, and pitting potential of MMCs was almost same as corrosion potential, while pitting susceptibility of MMCs was higher than that of AA2124 alloy. Galvanic current by formation of galvanic couple between SiCw and matrix reveals very low value because of large cathodic polarization of SiC. However, by increasing potential of matrix to pitting potential by this galvanic couple and thus, forming pits easily at the weak passive film near SiC reinforcing phase preferentially, it is concluded that pitting susceptibility of MMCs increases highly than AA2124 alloy of matrix composition.

The Effect of Tungsten Content on Nanocrystalline Structure of Ni-W Alloy Electrodeposits

The effects of tungsten content on the nanocrystalline structure of Ni-W alloy electrodeposits were investigated. Direct current electrodeposition was conducted with various W mole fractions in a nickel sulfate solution containing citric acid as a complexing agent. Current efficiency was an almost constant value of 52% up to W mole fraction of 0.58. However, it was sharply decreased owing to severe H2 evolution with increasing cathode overvoltage at a W mole fraction higher than 0.74. The X-ray diffraction peak was decreased and broadened with increasing W content, and an amorphous pattern appeared clearly at 49 wt.% W. It was confirmed that Ni-W alloy deposit was transformed from crystalline to amorphous structure between 41 and 49 wt.% W. This result was also verified through a TEM analysis.

Characteristics of Indium Oxide Rod-Like Structures Synthesized on Sapphire Substrates

In2O3 materials consisting of dense arrays of vertically aligned rod-like structures were deposited on sapphire substrates by thermal chemical vapor deposition (CVD) using triethylindium (TEI) and oxygen as precursors at a substrate temperature of 350 oC. The rod-like structure with a triangular cross section had a cubic structure, exhibiting preferred crystallographic orientation in the [111] direction. The photoluminescence spectra of In2O3 structures under excitation at 325 nm revealed a visible emission.

Plasma cleaning of carbon species for silicon homoepitaxial growth

As device dimensions are reduced into the sub-micron region in pursuit of higher integration density and better circuit performance, low temperature processing, including low temperature cleaning and low temperature epitaxial growth, is becoming important. In order to achieve the high-quality epitaxy with the low temperature processing, the technique of in-situ plasma cleaning has been developed [1, 2]. Although there have been some reports on the plasma cleaning of surface oxygen prior to epitaxial growth, there are rare reports on the systematic study on the removal of surface carbon. Carbon is much more tenacious and stable on the silicon surface than oxygen and temperatures over 1100 ◦C are required to remove carbon from the surface by thermal desorption process. Here, we have performed the low temperature in-situ cleaning in order to reduce the interfacial carbon concentration. Substrates were 4 inch, czochralski-grown, p-type (100) silicon with 0.5–20 -cm resistivity. The wafers were RCA cleaned and HF dipped for 20–30 s in 10:1 aqueous solutions and rinsed in DI (deionized) water and then dried by blowing nitrogen on them. All the processes were done inside the class 100 cleanroom and it took only 10 s to load the wafer into the load lock chamber of the CVD reactor after the wafer was blowdried. After the wafers were transferred and loaded onto the heater stage, the main chamber was pumped down and ultimately 1–2 × 10−8 Torr could be attained. In-situ predepositon wafer cleaning was done by using ECR hydrogen plasma. The ECR hydrogen plasma is able to deliver a higher density of low energy and light hydrogen ions to the wafer, resulting in highly efficient cleaning without substrate damage [2]. The ECR chamber was at the side of the CVD chamber. ECR was operated at the 2.45 GHz S-band microwave frequency. Depositions were done by flowing 10 sccm SiH4 without carrier gases, immediately after the plasma was extinguished. We have applied the in-situ cleaning condition with a microwave power of 300 W, DC bias of 10 V, pressure of 1 mTorr, and the cleaning temperature of 600 ◦C and subsequently have deposited the silicon epitaxial layer. Fig. 1 shows the XTEM image of silicon epilayer and the interface, revealing that the high-quality epitaxial layer is produced by applying the in-situ plasma cleaning. Fig. 2 shows the SIMS depth profile

Passive-Active Transition Behavior of 304 Stainless Steel

The passive-active transition behavior of a 304 stainless steel was investigated by observing the self-activation behavior and nano-scale galvanostatic cathodic reduction experiment. The self-activation time, τ0 was dependent remarkably on concentration of sulfuric acid. It was appeared that applied nano-scale cathodic current density dissolved the passive film on a 304 stainless steel surface and shortened the activation time in galvanostatic cathodic reduction experiments. The applied cathodic current density was proportional to the reciprocal of activation time. From this linear relationship, the rate of the self-activation process, i0 was obtained. The i0 increased with increasing H2SO4 concentration. And i0 also increased with increasing passivation potential and passivation time. The stability of passive film increased in accordance with increasing τ0 and decreasing i0. Therefore, it was concluded that the stability of passive film on austenitic stainless steel is evaluated by the kinetic parameters of the self-activation rate i0 and the self-activation time, τ0.

Microstructure and Mechanical Properties of Si3N4/hBN Composites Prepared by Hot-Pressing Using Nitrided Si3N4 Powders

In order to produce Si3N4/hBN composite with low cost, it seems necessary to use nitrided Si3N4 powders since the cost of Si powder is much cheaper than that of Si3N4 powder. The purpose of the present work is to investigate the nitride conditions, and in particular, we focused on the relationship between microstructures and mechanical properties of hot-pressed Si3N4/hBN ceramic composite using nitrided Si3N4 powders. The mixed powders of Si3N4 and hBN were prepared by nitriding Si powders at 1380oC for 24 h, and subsequently sintered by hot-pressing at 1800oC for 2 h in N2 atmosphere. The microstructure and mechanical properties of the Si3N4/hBN composites were investigated. Flexural strength, Young’s modulus, and hardness decreased by the addition of 20 vol% hBN. The addition of BN resulted in a decrease in the modulus as well as an increase in the size of fracture source, both contribute to the observed decrease in mechanical properties. The Si3N4/BN based ceramic composites revealed enhanced crack deflection. The Vickers indentation crack paths in specimens are sinusoidal due to pull-out of grains during crack propagation.

Preparation of Porous Alumina Ceramics by Spark Plasma Sintering

Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.